CONTROL OF CORAL RF:EFS BY BORING SPONGES 47 



conical detritus heaps where the shcle masses have come to rest. 

 We have no direct eN'idence on how such movements originate; it 

 is probable that they are set off bv storms when the surge depth is 

 greater than normal. Because deep-water coral associations do not 

 build wave-resistant reefs, the combined effects of unusually heavy 

 wave action and structural weakening by boring sponges are likely 

 to have catastrophic consequences: under such conditions, corals 

 cascading down in large numbers may spread destruction rapidly 

 by means of successive collisions that culminate in major slides and 

 avalanches. 



Subsidence and Shnnj) 



This form of reef erosion differs from slides in that it is a slow 

 subsidence of intact reef masses over vertical distances as great as 

 20 meters. The unstable region is usually delimited by large cracks 

 along planes of slippage. Differential movement occurs by repeated 

 block fracture, resulting in the formation of slump terraces, some 

 of which are shown on the fathometer profile B in Fig. 10. Reef 

 slump in Jamaica is most common at depths below 20 meters where 

 vigorously growing fore-reef communities are perched on the edge 

 of a drop-off, and large, poorly consolidated coral promontories are 

 built out over the edge with their bases resting on unstable fine sedi- 

 ments. At such depths cementation by algae is slow, whereas boring 

 sponge activity is intense; hence the coral frame is exceedingly 

 fragile. We have no information as to the nature of the forces that 

 trigger off subsidence: on the one hand, it may be due simplv to 

 the progressive accumulation of an unstable mass of coral; on the 

 other hand, earthquake shocks may cause initial slip along planes 

 of weakness. 



Sediment Flow 



Besides coarse talus, reef associations also produce vast quantities 

 of fine skeletal debris that is winnowed out and deposited in thick 

 sand beds outside the community proper. In the turbulent shallow 

 zones the sediment is coarse and well sorted, but in calm deep water 

 there is less sorting and a large proportion of mud is present. Drain- 

 age of the fine calcareous residue from reefs is essential to the well- 



